The present invention relates generally to a method of controlling the viscosity of a cementitious mixture which is useful in applying the cementitious mixture.
Portland cement is one of the most widely used materials in the construction industry. The term “portland cement” is typically used to describe the kind of product obtained by heating lime-clay mixtures, or natural cement rock, to such a temperature that practically the entire product is sintered followed by grinding. The product is often a mixture of dicalcium and tricalcium silicate with lesser amounts of aluminate. Various additives can be included in accordance with conventional portland cement manufacturing practices. Portland cement sets rapidly, within about 24 hours, and continues to harden slowly over a period of years.
In many instances, it may be desirable to combine a portland cement or other cement with an aggregate material (e.g., sand, stone, gravel, pebbles, granite, carborundum, marble chips, and mica). For example, conventional “concrete” is a cementitious mixture comprising portland cement, water and aggregate, such as sand or a mixture of sand with gravel. The particle sizes for the cement powders are relatively small compared to the finest aggregates (e.g., cement particles typically have a surface area of about 0.5 to about 3.5 m2/g, measured by B.E.T. whereas sand grains are typically about 0.08 to about 5 mm in diameter). In conventional concrete compositions, the aggregate constitutes more than about 10 percent by weight of the cementitious mixture and usually from about 15 to about 30 percent by weight of the cementitious mixture. The cement is the matrix that, when mixed with water, hardens and bonds the aggregate particles into a rigid solid.
The viscous properties of a cementitious mixture have certain undesirable characteristics, for example, being fluid-like in their working state; cementitious mixtures tend to sag, run or slump during and immediately after application. A layer of concrete which is spread on a sloping surface tends to sag toward the lower end. Cementitious mixtures sprayed or layered onto a vertical surface (e.g. stucco) tend to sag or run if the applied layer is too thick. Cementitious mixtures applied to a ceiling, roof or overhead structure may sag or detach, even if supported on a frame, if applied too thick. The surface of cementitious materials may not faithfully retain patterns on their surface due to running before hardening. Reinforcement materials, such as steel bars, may be difficult to coat. A cementitious mixture, when sprayed, has a tendency to spatter, rebound or bounce off the surface to which it is being applied, making the application inefficient. In these aforementioned examples, it would, in many cases, be advantageous for the cementitious mixture to have a higher working viscosity.
Typically, the method of increasing viscosity is to add less water to the cementitious mixture. This option, however, significantly decreases the working time of the cement. Decreased water premix concrete, for example, could harden before delivery. There are also distinct disadvantages in the workability of concrete mixed, transported and applied entirely in a decreased water or viscosity-enhanced state. For example, pumping or spraying of viscosity-enhanced premix concrete is more difficult. Additionally, if insufficient water is added, the final mechanical properties may be adversely impacted (e.g. modulus of elasticity, tensile strength, crack resistance, wear resistance).